1. Field of the Invention
This invention relates to an electronic apparatus having a microprocessor unit (MPU) and a plurality of read-only memories (ROMs) having common address regions.
2. Description of the Prior Art
There have been a large number of apparatuses using MPUs. The control circuit for each of these apparatuses comprises in gross a MPU, a ROM, a random access memory (RAM), a timer, an I/O port, a driver and the like. Among these components, the ROM is used as a main unit for controlling the apparatus. Its storage contents include various elements, such as control codes for the apparatus, a character generator, tables and the like.
In an electronic apparatus having a ROM, when control codes, a character generator, tables and the like are stored in the ROM and the apparatus is operated, if there is any bug in a part of these elements, there may arise a need to deal with the bug by promptly correcting in case the number of the bugs is large.
If there is any problem, for example, in a control code, a case may arise in which the apparatus can not perform a normal operation and causes excursion. If there is any bug in a character generator, troubles, such that correct characters are not output, may arise. Therefore, prompt countermeasures are required, particularly, for example, after the apparatus has been shipped to the market. As a countermeasure, in most cases, a ROM in which a problem has occurred is replaced with a new ROM in which the problem has been corrected.
On the other hand, in accordance with the recent trend toward a large-capacity ROM, control codes, a character generator and the like, which have previously been stored in separate ROMs, now can be stored in a much smaller number of ROMs by storing a certain group of these elements in a single ROM, or even all of these elements. Furthermore, ROMs have been modified by changing masks because of the absence of erasable programmable ROMs (EPROMs) having large capacity and the need for cost reduction. Since a drastic cost reduction and a reduction in the size of a substrate due to a reduction in the number of components can be expected, as described above, ROMs having still larger capacity have recently been used.
Under such circumstances, however, if there is any bug in a ROM as described above, there are problems in that no EPROM for replacing the ROM exists if the ROM produced by certain masks has a large capacity, and about two months are needed for providing a new ROM by changing masks. Hence, a prompt countermeasure cannot be taken.
If an EPROM having large capacity existed, the ROM would be able to be promptly replaced with the EPROM. However, the cost for the EPROM is much higher than the cost for a masked ROM. Hence, the cost for the replacement drastically increases.